There should be 3 wires going to the alternator. All of which need to be connected for proper operation. First is the large red wire, Bat + then there should be a blue wire connected to the case and the engine block, this Bat - or ground, must be grounded due to the mount bushings on the alternator, and finally a small exciter wire which is the one that lights the idiot light, I suspect that your small wire is disconnected some where along the way before it hits the idiot light. Happy hunting.
Chuck Jaxel